In general, battery state of health defines the performance of a battery relative to its original condition. Changes in the state of health from the original condition typically signals performance loss due to aging processes or a battery's end-of-life. Historically, state-of-health methods have been developed to predict the remaining life of valve-regulated lead-acid rechargeable batteries in combustion engine vehicles. Recently, the focus has shifted towards state of health methods for rechargeable batteries used in hybrid electric vehicle and electric vehicle applications. In the emerging markets, battery usage history is coupled with physical metrics of electrical current and operating temperature to estimate the remaining usage life of the battery.
Traditional methods for battery state of health typically related to capacity evaluation and the state of the batteries' remaining performance life. These methods involved discharging batteries at constant current to predefined voltages, which is expensive, time consuming, and requires the batteries under test to be out of service for extended periods of time. Traditional commercial state of health testers measure impedance by applying an AC current at a single-frequency or as a square-wave pulse. The frequency at which each tester operates varies depending upon the manufacturer. See, e.g., Hewlett-Packard, 1000 Hz; Biddle Instruments, 60 Hz; Celltron, ˜10 Hz; and Midtronics, 25 Hz. The output values are also specific to the manufacturer. Each of these testers operates at high frequency where the battery impedance is nearly the internal resistance value. The internal resistance value was found to have strong relationship with the remaining capacity of the battery. In short, the earliest uses of state of health monitor were to relate impedance and remaining useful life.
Lithium-ion batteries are state-of-the-art rechargeable power sources for high energy electronic devices such as missile systems, torpedoes, communication equipment, and night vision goggles. Recent safety incidents involving thermal runaway reactions in lithium-ion batteries have arisen resulting in fire and the subsequent loss of equipment and resources. Safety of lithium-ion batteries is critical to fulfill many types of activities, including military operations. Overcharging lithium-ion batteries decreases their capacity and cycling performance and increases the risk for venting toxic materials, fire and explosion.
To avoid overcharging lithium-ion batteries, information must be known about the type of lithium-ion batteries being utilized. Specifically, the upper and lower voltage limits for charging are specific to the chemistry of the positive/negative electrode couple and electrolyte materials. The most common Li-ion battery cathode material is LiCoO2 which has an upper voltage limit of 4.2 V and a lower limit of 2.8 V vs. Li+/Li0. Charging above the 4.2 V cutoff voltage, or overcharging, is often the result of improper charging practices, faulty cell balancing electronics in multi-cell batteries, thermal imbalances in battery packs, and/or cell manufacturing defects. The direct path from overcharge abuse to thermal runaway is not entirely understood, but is believed to comprise the following steps: positive electrode materials undergo structural transformations and release oxygen during overcharge, where the greatest quantities are evolved with voltage, temperature; and the oxidation ability of the active material resulting in rapid exothermic reactions with the electrolyte solvent. The release of gaseous species causes pouch swelling, venting or even rupture of the cell packaging which is dictated by the severity of the overcharge current and voltage. At potentials >4.5 V organic electrolytes decompose forming insoluble products which block the pores of the electrode and cause gas generation in the cell. Abusive charge practices (overcharge and high rate charging) result in plating lithium metal on the anode surface. These lithium defects may grow with abusive charge cycling resulting in either an internal short circuit or a violent reaction between the overcharged anode and electrolyte. Overcharge of just one cell is significant since a failure resulting in explosion or fire can easily cascade to other cells in a battery pack.
There are no prognostic methods for lithium-ion batteries to detect degradation or predict an imminent failure. Health monitoring and prognostics for Li-ion are limited to state-of-charge determination, voltage estimation, capacity estimation, and RUL prediction. One proposed health monitoring method calls for periodic voltage and temperature readings transmitted through the use of an RFID (radio frequency identification) tag. These traditional methods lack the specific ability to diagnose the degradation processes that occur due to abuse (either internal or external) and that seriously affects the ability of the battery to perform in a safe manner.
Accordingly, there remains a need in the art for an impedance diagnostic method for monitoring the state-of-health of rechargeable lithium ion batteries and diagnosing battery defects. The diagnostic method should be able to act as a diagnostic to determine the health of lithium-ion batteries and a prognostic for battery failure. The ultimate purpose of the method is to identify defective batteries to be taken out of service before causing a serious safety hazard.